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Cyclic AMP-stimulated protein kinases at brain synaptic junctions.

P T Kelly, C W Cotman, M Largen

    The Journal of Biological Chemistry
    |March 10, 1979
    PubMed
    Summary

    Synaptic junctions exhibit high cyclic AMP-dependent protein kinase activity, phosphorylating specific proteins. This activity is crucial for synaptic function and involves identified regulatory subunits.

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    Area of Science:

    • Neuroscience
    • Molecular Biology
    • Biochemistry

    Background:

    • Cyclic AMP (cAMP) is a crucial second messenger in neuronal signaling.
    • Understanding cAMP-mediated phosphorylation in synaptic structures is key to elucidating synaptic plasticity and function.

    Purpose of the Study:

    • To investigate endogenous cAMP-stimulated protein phosphorylation in rat brain subcellular fractions.
    • To identify specific synaptic loci and protein substrates involved in cAMP-dependent phosphorylation.
    • To characterize the protein kinase activities within synaptic junctions (SJ).

    Main Methods:

    • Fractionation of rat brain into synaptic plasma membranes (SPM), synaptic junctions (SJ), and postsynaptic densities (PSD).
    • Assay of protein kinase activity using endogenous and exogenous (histone) substrates.
    • Measurement of cAMP-stimulated phosphorylation and identification of phosphorylated proteins using molecular weight (Mr).
    • Photoaffinity labeling with [32P]8-N3-cyclic AMP to identify regulatory subunits.

    Main Results:

    • SJ fractions showed the highest endogenous protein kinase activity, significantly exceeding SPM and PSD fractions.
    • cAMP (5 µM) increased overall phosphorylation in SJ fractions by 36%, with specific proteins (Mr 85,000, 82,000, 78,000, 55,000) showing 2- to 3-fold increased incorporation of phosphate.
    • Two SJ polypeptides (Mr 55,000 and 49,000) were identified as potential regulatory subunits of cAMP-dependent protein kinases.
    • A heat-stable inhibitor protein significantly reduced cAMP-independent kinase activity, suggesting involvement of the catalytic subunit of cAMP-dependent kinase.

    Conclusions:

    • Synaptic junctions are primary sites for endogenous cAMP-dependent protein phosphorylation in rat brain.
    • Specific synaptic proteins are directly phosphorylated in a cAMP-stimulated manner, implicating them in synaptic signaling pathways.
    • The identified polypeptides represent regulatory subunits of cAMP-dependent protein kinases, crucial for synaptic function and plasticity.

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